PROJECT SUMMARY Erythropoietin (Epo) and its receptor EpoR are primary regulators of erythropoiesis. Normal Epo levels support basal erythropoiesis. In anemia, blood loss, or hypoxia, Epo levels rise, initiating signaling pathways to expand erythroid progenitors, increasing red cell production during the erythropoietic stress response. In published work, we showed that the constitutively active JAK2 mutant JAK2(V617F), found in myeloproliferative neoplasms, needs to associate with the EpoR to transmit oncogenic signaling in erythroid progenitors to drive erythrocytosis. While EpoR signaling in more differentiated erythroid precursors has been extensively studied, EpoR signaling in early progenitors, which controls stress erythropoiesis and erythrocytosis, has not been elucidated. In preliminary studies, we show that JAK2(V617F) oncogenic signaling requires the distal EpoR cytoplasmic domain. Because this distal EpoR domain is essential for stress erythropoiesis but dispensable for basal erythropoiesis, JAK2(V617F) may hijack EpoR- dependent stress erythropoiesis pathways to drive erythrocytosis. Using a new flow cytometry strategy, we identified a previously unrecognized population of early colony- forming erythroid progenitors (CFU-E), which we named stress CFU-E or sCFU-E, as a specific target of both the JAK2(V617F)-driven signals and the stress erythropoietic response. Elucidating EpoR signaling that supports sCFU-E expansion may reveal new therapeutic avenues that can target erythrocytosis while sparing normal erythropoiesis. The central goal of this proposal is to characterize the EpoR-dependent mechanisms that drive sCFU-E expansion in stress erythropoiesis and in JAK2(V617F)-driven erythrocytosis. Aim 1 determines mechanisms and EpoR signaling in sCFU-E expansion. Aim 2 leverages our new RNA-seq data to examine novel regulators downstream of EpoR in stress erythropoiesis, murine erythrocytosis and human polycythemia vera. Together, these results will elucidate mechanisms controlling stress erythropoiesis and pathological erythrocytosis, and may lead to novel therapeutic interventions.